It is generally well-known that silver surfaces even when not in use get tarnished in course of time due to corrosive reactions. The same phenomenon can be observed when silverware is washed in machine dishwashers. This is due to several chemical reactions which can occur when the silver gets into contact with sulfur, oxygen and chlorine containing compounds under the high temperature and alkalinity conditions in a machine dishwasher. The sulfur containing compounds result from food residues, e.g. egg yolk, which are solved in the dishwashing water. The oxygen containing compounds reactive with the silver surfaces, e.g. peracetic acid, can be formed when bleach activators are used in the dishwasher cleaning composition. A high amount of salt in dishwashing water can result in chlorine containing plaques on the silver surfaces.
Several silver corrosion protection agents have been described in the patent literature. The British patent GB 1131738 discloses dishwashing agents which use benzotriazoles as a corrosion inhibitor for silver. Benzotriazoles in the context of silver corrosion protection are also disclosed in the U.S. Pat. No. 2,549,539 and the European patents EP 135 226 and EP 135 227.
Another group of compounds used as silver corrosion protection agents comprises manganese salts or manganese complex compounds. The German laid open patent number DE 4315397 discloses organic and anorganic redox compounds containing manganese(II) compounds, e.g. manganese(II)sulfate, manganese(II)acetoacetate and manganese(II)acetylacetonate. These low valent manganese compounds have to be coated prior to their use in cleaning compositions containing bleaching agents in order to avoid their oxidation or decomposition during storage.